In multi-user wireless communication systems, such as wireless local area networks, satellite communications and mobile phone networks, multiple transmitters and receivers may communicate simultaneously through a common wireless communication medium. One communication format with attractive features for high data rates and low power consumption is ultra wideband (UWB), also known as impulse radio (IR), in which the transmitter generates a train of ultra short pulses spreading the energy of the transmitted signal across an ultra wide bandwidth. IR technology employing time-hopping (TH) codes pseudo randomly spreads pulses in time, and is often referred to IR multiple access (IRMA).
In general, IRMA applies a TH code chosen from a set of orthogonal “spreading codes” to an outbound stream of “symbols.” Each IRMA pulse transmits a “symbol” representing a discrete information bearing value selected from a finite set (“alphabet”). For example, simple alphabets used by transmitters may be {+1, −1} or {−3, −1, +1, +3}. TH codes apply a finite set of integer values to time shift a sequence of transmitted pulses and produce a set of “chips” for each value to be transmitted. Each TH code has a sequence, resulting in each IRMA pulse train having a defined time period. The resulting chips are transmitted according to some modulation scheme, such as pulse position modulation (PPM). In order to separate signals from multiple users, the receivers isolate the signal of the desired user by matching the user's signal to the corresponding orthogonal spreading code associated with that user.
IRMA systems often operate in dense multi-path environments. IR signaling provides resolvable multi-path components and enables collecting energy from multi-path propagation with appropriate receiver design. However, when multiple users are present, the multi-path propagation also induces multi-user interference (MUI). In addition, the communication channel between the transmitter and the receiver can also become “frequency selective” in that certain frequencies exhibit fading, i.e. significant loss of signal. Consequently, inter-symbol interference (ISI) in which the transmitted symbols interfere with each other destroys the orthogonality of the waveforms at the receiver. MUI, together with ISI, can cause the receiver to be unable to correctly separate the multi-user waveforms, eventually leading to data loss and/or bandwidth and power inefficiencies.
Various “multi-user detectors” have been developed for separating non-orthogonal UWB multi-user waveforms. These multi-user detectors, however, are often complex and expensive to implement in typical mobile communication devices, and typically require knowledge of the characteristics of the current communication channel. For example, linear detectors, such as decorrelating and minimum mean-squared error (MMSE) detectors, often require inversion of large matrices with size increasing in proportion to the square of the number of users, while optimum maximum likelihood (ML) detectors entail exponential complexity. In addition, some analog IRMA systems approximate MUI as Gaussian noise and attempt to suppress it statistically. Such systems require successful application of strict power control and rely on the Gaussian approximation. However, when the number of users is not sufficiently large, the Gaussian approximation is not valid.